Transition metal ions are biologically required yet toxic when in excess. As such, prokaryotes have developed metal homeostasis systems that allow for the acquisition of essential metal ions, the delivery of these metal ions to target proteins, and the export of metal ions from the cell. Specialized metal‐binding proteins termed
metallosensors
bind to a cognate metal ion(s) in the cell and either transcriptionally repress the expression of importers or de‐repress the expression of exporters or sequestration systems and therefore govern cellular metal homeostasis. Metallosensors must be selective and appropriately sensitive toward the binding of their cognate metals. Prokaryotic metallosensors have been identified in ten different protein structural families. Each family exploits either one general metal‐binding‐site region, individual members of which have evolved different coordination sites, or alternatively, employs multiple metal‐binding sites to effect coordination of different metal ions. There is now broad support for the hypothesis that metal responsiveness in metallosensors is most closely linked to the coordination number and geometry adopted by cognate metal ion(s), and this is the subject of this article. Since a range of metal ions can be collectively sensed by a single repressor family, a particular protein fold cannot be specific for one particular metal ion. In fact, the converse is true: nature has used convergent evolution to evolve metal‐specific sensors on a variety of structural scaffolds that exploit common principles of coordination chemistry (ligand type, coordination number, and geometry) to effect the same biological outcome.